DB<120> $SIG{__WARN__} = sub { print "<<<$_[0]>>>" }

  DB<121> warn "xxx" 
<<<xxx at (eval 58)[multi_perl5db.pl:644] line 2.
>>>

##</code><code>##

  DB<133> $x =~ s/ (?{ print "\$2 undefined" unless $2 })/ /g
$2 undefined